Recently, lithium ion batteries are used as a power source for a portable device such as a portable telephone or a digital camera because of requirements for higher capacity and miniaturization. Lithium ion batteries are also used as a power source for power-assisted bicycles or power tools because of their high energy density and because they have no memory effect. Further, stacked lithium ion batteries that includes an electrode having a laminate structure and that use an aluminum laminate film as exterior covering the battery have been commercialized because these batteries provide advantages such as lighter weight and improvement of heat dissipation, and serious research has focused on developing a battery pack or a battery system in which multiple lithium ion batteries are series or parallel connected.
FIG. 1A is a plan view schematically showing a typical stacked secondary battery that includes an aluminum laminate film as an exterior member. FIG. 1B is a schematic view showing a cross section taken along the line D-D′ shown in FIG. 1A (and being parallel to a stacking direction), and specifically shows a sectional structure near the collective tab of a conventional stacked secondary battery.
Positive electrode collector 111 includes active material forming region 111a to which positive electrode active material 112 has been applied, and positive electrode collective tab 111b as a region to which positive electrode active material 112 has not been applied. Positive electrode collective tab 111b is projected from the end of active material forming region 111a to be pulled out from active material forming region 111a. Negative electrode 120 also includes a negative electrode collective tab disposed to be pulled out from negative electrode active material 122.
In the case of a lithium ion secondary battery, positive electrode collector 111 is made of aluminum foil, and positive electrode active material 112 is made of lithium-containing oxide such as lithium cobalt oxide, lithium nickel oxide, or lithium manganese oxide. Negative electrode collector 121 is made of copper foil, negative electrode active material 122 is made of carbon, and separator 130 is made of a porous plastic film.
In such a lithium ion secondary battery, a short circuit between the electrodes in the battery is a major problem. The reason for this is that when the electrodes are slightly short-circuited, self-discharge of the battery is accelerated, and that when the electrodes are largely short-circuited, heat is generated, which leads, in the worst case, to the occurrence of smoke or fire, creating a dangerous situation.
A maximum short-circuit current flows between the electrodes when the metals of negative electrode collector (copper foil) 121 and positive electrode collector (aluminum foil) 111 come into contact with each other. However, the stacked secondary battery is designed in such a way that, for structural and manufacturing reasons, such contact will not occur. In other words, positive electrode collector 111 and negative electrode collector 121 are arranged to be spatially separated from each other by separator 130 to prevent internal short circuit.
A region through which current is likely to flow next is a region between positive electrode collector 111 and negative electrode collector 122, i.e., a region where region 111b in which positive electrode collector 111 is exposed faces the negative electrode active material via separator 130. In such a region, there is a possibility that during battery use, due to external factors such as expansion, vibration, or shocks to the electrodes caused by repeated charge and discharge, active materials 112 and 122 will shed from electrodes 110 and 120, and pass through separator 130, thereby generating short circuit between the electrodes. The same phenomenon may occur between negative electrode collector 121 and positive electrode active material 112.
Thus, in the stacked secondary battery, to solve the problem caused by shedding of the active materials, a safer structure must be designed between the electrodes. For example, Patent Literature 1 proposes, in a secondary battery in which electrode elements have winding structures, a method for preventing internal short circuit even when shed active materials pass through the separator by having tape adhere to a region where the positive electrode collector is exposed, i.e., a region that faces the negative electrode active material via the separator. Patent Literature 2 proposes, in a wound secondary battery, a method for preventing internal short circuit, that is caused by shedding of active materials, by heat-treating a porous separator disposed between electrodes so that the separator becomes nonporous